Networks of a first type are known in which information is routed between different elements of the network by using static routing tables. For each element of the network, a routing table lists the portions of the network that are directly or indirectly connected to the network element in question, together with the interfaces of said network element giving access to these various portions of the network. A routing table is said to be static when any change to the network assumes that a corresponding change will be made by an operator to the routing tables of the network elements.
Networks of a second type are known in which information is routed between the various elements of the network by using dynamic routing tables. One such routing protocol is the link state hierarchical routing protocol, known as open shortest path first (OSPF), which constitutes specifically the subject matter of the Internet Engineering Task Force (IETF) technical report RFC 2328. This type of routing protocol provides for a state in which identification messages and routing table update messages are interchanged. Each identification message includes a neighborhood identification field normally dedicated to receiving the identifiers of neighboring network elements that are connected directly to the network element issuing the identification message, and serving to inform the neighboring network elements that the emitting network element is active. The update messages contain, in particular, in usual manner, the identifiers of network elements to which the message-emitting network element knows it is connected either directly or indirectly.
There are proposals for installing on-board networks in certain airplanes, comprising an avionics network of the first type that is secure and that is used to operate the airplane, and a public network of the second type that is less secure and that is used by the crew for obtaining information about the flight or the passengers, about the airports used, . . . , and also to enable passengers to have access to entertainment means. The public network is also used by maintenance teams on the ground for obtaining flight parameters after the airplane has landed. The avionics network is connected to the public network via at least two first routing elements selectively associated with the avionics network, and at least one second routing element associated with the public network and connected to the first routing elements.
Ideally, in order to ensure full inter-operability between the two networks, it would be necessary for the first routing element to be capable of implementing the routing protocol on the basis of dynamic tables and of exchanging identification messages and update messages with the second routing element. Nevertheless, such exchanges of information would burden the first routing devices in quite significant manner, particularly since as a general rule such routing devices have computing resources that are relatively limited.
In addition, in order to ensure that the network is reliable, provision is made for the two first routing elements to be connected to the first network in alternation. It is thus essential for the second routing elements to be able to identify which first routing element is active in order to perform effective routing.